Solar DC cable is sized on voltage drop, not just current — and at 12, 24 and 48 volts that is unforgiving, because the allowed drop is tiny. A wire that would be wildly oversized on 120/240 V AC is often the bare minimum on a battery run. Here is how to size it.
The formula
Required area (mm²) = 2 × 0.0175 × current × one-way length ÷ allowed volt-drop, then round up to the next standard gauge. 30 A over 16 ft (5 m) on a 12 V system at 3% (0.36 V allowed) needs about 14.6 mm² — so 6 AWG (16 mm²). Enter the one-way route distance, not the loop — the formula already doubles it for the return conductor.
Solar Cable Size Calculator
Enter system voltage, current and run length for the copper gauge that holds your volt-drop.
Why low voltage punishes thin wire
The allowed drop is a percentage of a small number: 3% of 12 V is just 0.36 V, versus 3.6 V on a 120 V circuit. And the same power means far more current at low voltage. Doubling the system voltage to 48 V quarters the current and slashes the copper needed — the main reason serious systems run 48 V.
What drop to allow
| Run | Allowed drop |
|---|---|
| Panel → controller | up to 3% |
| Battery → inverter | 1–2% |
Keep battery-to-inverter cables as short and fat as practical — that is where high currents flow and every volt of sag hits the inverter’s low-voltage cutoff early. It is the run where systems most often disappoint.
Volt-drop isn’t the only check
This sizes purely on voltage drop — you must also confirm the wire’s ampacity (the NEC tables) and fusing. At solar currents the volt-drop size is usually the larger of the two, but always fuse battery cables at the battery end, use proper DC-rated wire (double-insulated PV wire on the roof) and crimped lugs. DC arcs don’t self-extinguish like AC.